Hydrogen sulfide is a major source of pollution of gas streams since it is liberated as a waste by-product in a number of chemical processes, such as sulfate or kraft paper pulp manufacture, viscose manufacture, sewage treatment, the production of organic sulfur compounds, as well as during petroleum refining and in the production of natural gas and combustible gases from coal, such as in coking operations. Hydrogen sulfide is also present in geothermal steam, which is captured for use in power generating plants.
To eliminate these polluting sulfur gases the art has developed several oxidation-reduction (“redox”) processes that use an aqueous chelated metal catalyst solution for removing hydrogen sulfide from a gas stream. In those prior art processes a hydrogen sulfide-containing gas, known as “sour gas,” is contacted with a chelated metal catalyst to effect absorption. Subsequent oxidation of the hydrogen sulfide to elemental sulfur and concurrent reduction of the metal to a lower oxidation state also occurs. The catalyst solution is then regenerated for reuse by contacting it with an oxygen-containing gas to oxidize the metal back to a higher oxidation state. The elemental sulfur is continuously removed from the process as a solid product with high purity. Illustrative, but not exclusive, of these oxidation-reduction processes is the description contained in U.S. Pat. No. 4,622,212 and the references cited therein.
In order to return the “spent” liquid redox catalyst solution to its original oxidation level so it can be recycled for subsequent use in the process, oxygen must be supplied to the spent redox catalyst solution. This is typically accomplished using an oxidation process where various mechanical apparatus, including well-known tank spargers, use compressed air as the source of oxygen. Typically, such oxidation processes are operated at pressures lower than the pressure of the reduction portion of the process, i.e., the absorber, more typically at about atmospheric pressure. Use of low pressure oxidizers are a result of an attempt to minimize capital costs by eliminating the need for more expensive high pressure equipment. Although initial capital cost of equipment may be lower, operating at a large pressure differential between the absorber and the oxidizer has a host of other inherent problems. For example, in these previously known processes, the higher pressure redox solution exiting the absorber must be reduced in pressure before entering the oxidizer. This is typically accomplished through a flash drum or a series of flash drums. Reducing pressure of the redox solution has unfortunate consequences, such as foaming, lost of gas product, and rapid erosion of control valves due to the suspended solid sulfur particles. All of these problems reduce the overall process economics and the operability of the process.
Up until now, the art has failed to come up with a high pressure reduction oxidation process that eliminates the above problems, yet still provides a cost effective process for the removal of sulfur from hydrocarbon process streams. These and other advantages will become evident from the following more detailed description of the invention.